The present invention is generally related to temperature measuring devices, and more particularly, to an apparatus for measuring temperature by collecting infrared radiation from a target and converting the infrared radiation into a signal proportional to the target temperature. In one embodiment, the present invention operates as a clinical thermometer for the measurement of human body temperature.
It has long been the interest of medical professionals to measure and monitor the body temperature of their patients. This interest is primarily based on the fundamental relationship existing between the pathologic state and internal body temperature. Many illnesses are characterized by a deviation from normal body temperature, and the success of certain medical regimens, e.g., antibiotics, is best tracked by directly monitoring the body temperature and its response to the regimen.
In the past, mercury thermometers have been predominantly relied upon for the measurement of body temperature. Although the mercury thermometer has enjoyed universal use, it suffers from several key drawbacks. The mercury thermometer as a contact sensor, takes several minutes to equilibrate in temperature with the contacted tissue. This equilibration time is a significant cost factor when patients number in the hundreds as in hospital wards. Also, the contacted tissue may be and often is a source of infectious bacteria and viral agents, thus necessitating additional time for sterilization of the thermometer between readings. In addition, mercury thermometers are most often contacted to mucous membranes in the mouth or rectum; both locations have been found to be less than perfect predictors of the internal body temperature of the patient.
Electronic thermometers using a permanent thermistor probe with a disposable probe cover have replaced many of the glass thermometers, especially in hospitals. Those thermometers have the advantage over glass thermometers in that they give a reading in about 40 seconds and in that the reading is digital. But, in 40 seconds thermal equilibrium has not yet been reached with the thermistor thermometers generally used. The thermometer electronics provides a correction by interpolating the time-temperature curve to "predict" an estimated stabilized reading. This technique introduces errors and makes accuracy verification against a temperature standard difficult or impossible.
Some of the above problems have been alleviated by the use of thermocouple probes, which now can be made to be both disposable and characterized by a relatively short time constant. Still the thermocouple probe remains a device designed to contact the patient's mucous membranes with both the remote possibility of cross-contamination and less than ideal accuracy.
In the search for a better thermometer, it has become known that the tympanic membrane or the ear drum is characterized by an inherent temperature that is essentially identical to the internal temperature of the body. The tympanic membrane is both delicate (i.e., easily damaged) and sensitive, therefore making the application of a contact-type sensor to the tympanic membrane less attractive.
Non-contact sensing of temperature becomes possible through the application of infrared radiation sensor technology. Infrared radiation sensors such as the thermopile type, have found substantial use for temperature measurement of remote or environmentally isolated targets. For example, the temperature of stars, planets, and furnaces are all measured by sensing the emitted infrared radiation from the target surface. In these applications, the infrared radiation sensors have been found to have high accuracies within narrow temperature ranges. Accurate measurements within 0.1 degrees Fahrenheit have been attained.
The use of an infrared radiation sensor to measure the temperature of the tympanic membrane has been attempted. For example, U.S. Pat. No. 4,602,642, discloses a temperature sensor based on infrared radiation detection. This instrument is based on a design that is quite complicated. For example, a complex sensor heating apparatus is operated with a close loop temperature controller to hold the temperature of the thermopile detector constant near the patient's body temperature, i.e., 98.6.degree. Farenheit. In addition, the device requires frequent calibration, based on a microprocessor controlled calibration sequence, against a target heated to a controlled temperature. Also, the use of the instrument requires considerable training and skill, since it has to be very carefully pointed toward the tympanic membrane in order to give an accurate temperature read out. These complexities have made the device difficult and expensive to manufacture and unaffordable to the majority of users.
It was with this understanding of the problems associated with the prior art that the subject invention was made.